1. Field of the Invention
This invention relates to a deposited-film-forming apparatus for superposingly forming a plurality of different thin films on the surface of a belt-like member by plasma-assisted chemical vapor deposition (CVD). More specifically, it relates to an apparatus for continuously mass-producing photovoltaic devices such as solar cells by using a roll-to-roll system.
2. Related Background Art
With regard to photovoltaic devices, semiconductor layers which are important constituents thereof have semiconductor junctions such as pn junctions or pin junctions.
When thin-film semiconductors such as amorphous silicon (hereinafter, often a-Si) are used, material gases containing elements such as phosphine (PH.sub.3) and diborane (B.sub.2 H.sub.6) acting as dupants are mixed with a main material gas such as silane, and glow-discharge decomposition is effected to obtain semiconductor layers having desired conductivity types. It is known that these semiconductor layers are successively superposingly formed on a desired substrate, thus the above semiconductor junctions can be achieved with ease. Accordingly, to produce photovoltaic devices of an a-Si type, methods are proposed in which independent film-forming chambers for forming the respective semiconductor layers are successively provided and each semiconductor layer is formed in each film-forming chamber. In this connection, U.S. Pat. No. 4,400,409 discloses a continuous plasma-assisted CVD apparatus employing a roll-to-roll system.
As reported therein, according to this apparatus, a plurality of glow-discharge regions are provided and a sufficiently long, flexible substrate having a desired width is provided along the course where the substrate passes successively through the glow discharge regions. The substrate is continuously transported in its longitudinal direction while semiconductor layers with required conductivity types are deposited in the respective glow-discharge regions, whereby devices having semiconductor junctions can be continuously produced.
The above apparatus also utilizes gas gates which are used so that dopant gases used when the semiconductor layers are formed can be prevented from diffusing and mixing into other glow-discharge regions.
Stated specifically, the respective glow-discharge regions are separated from each other by slit-shaped separation paths, and means for flowing scavenging gas such as Ar or H.sub.2 through the separation paths are employed.
However, in such a conventional vacuum apparatus, a plurality of vacuum chambers are provided in a connected form when it is set up as a mass-production apparatus. Hence, the apparatus tends to have a large size and, in particular, to be very long in the transport direction of a belt-like member (substrate).
Thus, the apparatus for producing photovoltaic devices by the conventional roll-to-roll system type plasma-assisted CVD tends to be very long in the transport direction of the belt-like member. In addition, when attempting to obtain photovoltaic performance with desired characteristics, processes have been taken to raise the temperature of the belt-like member and to change the temperature for the baking of the vacuum chambers in order to remove impurities. However, the heat history due to the repetition of thermal expansion caused by such processes and contraction caused by cooling acts as stress in the vacuum chambers to bring about strain, in particular, to cause cracks at portions having a low strength, and has a possibility of causing a problem of leaking.